Improve Panstarrs cross matching

[twshimwell]

In offsets.py we attempt to match with Panstarrs to derive offsets per facet after removing extended sources from the LOFAR catalogues.

Current filtering in master branch is very crude (filter=(lofar['E_RA']*3600.0)<2.0, filter&=(lofar['E_DEC']*3600.0)<2.0, filter&=(lofar['Maj']*3600.0)<20). At low declinations and low galactic lattitude this filtering removes far too many LOFAR sources and the cross matching with Panstarrs doesnt work very well as a result.

In offsets_compact branch ive tweaked the filtering so that it corrects for radial smearing before finding an envelope to separate compact from extended sources (see attached plots - saved as "radial-correction.png" and "fitted-skew-envelope.png" in offsets.py). This leaves far more sources for cross matching compared to the old approach.